X-ray mammography and ultrasonography are the current clinical tools used to detect breast cancer. Mammography, however, is ionizing radiation, and imaging radiographically dense breasts is difficult. Ultrasonography cannot detect many of the nonpalpable cancers that are not visible on mammograms of good quality. To provide an additional non-invasive, non- ionizing, inexpensive diagnostic tool for breast cancer, we have created a novel detection system combining the penetration advantage of ultrasound with the contrast advantage of laser imaging: tissue imaging using ultrasound to modulate laser light transmission in biological tissues [Wang 1994a]. Our recent breakthrough studies have demonstrated for the first time that objects buried in turbid media can be imaged using ultrasound-modulated laser light. This research has indicated the possibility of clinical application in breast imaging and cancer detection. This technique detects tumors based on differences in mechanical and optical properties between normal and abnormal tissues. Continuous-wave ultrasound focused into the tissue modulates the laser light passing through the focal spot. The ultrasound-modulated laser light reflects the local mechanical and optical properties at the focal zone. Buried objects are located by scanning the device or the target tissue and detecting alterations of the laser light modulated by the ultrasound. Our specific aims in the present project include (1) refining our current instrument to better image objects buried inside tissue-simulating phantoms that are optically and mechanically similar to tissues; (2) in vitro imaging of animal tumors buried inside chicken breast tissues and in vitro imaging of mastectomy specimens; and (3) in vivo imaging of breast with and without tumors that have been imaged with current techniques in the clinic to verify this technique and to correlate with mammograms and ultrasonograms. Each technique of breast imaging measures different properties of tumors in breast tissue. If this research succeeds, ultrasound-modulated laser light imaging technique will certainly provide an excellent adjunct tool to mammography and ultrasonography for breast imaging and may even revolutionize medical imaging in general.